Mushroom Wall

Put it up yourself and it behaves properly: enemies will not spawn against it and a room built with it counts as housing. The version that appears when you spray mud or jungle background with Dark Blue Solution only looks the same. It stops nothing from spawning and it breaks into nothing when you knock it down.

This catches people converting a patch of jungle into mushroom for a Truffle house or a farm. The background flips along with the ground, and the room you were counting on quietly starts spawning things. Clear the sprayed walls and place real ones before you call the build done.